G10 STRUCTURE OF THE SPINAL CORD fell. XLII. 



Tho general rule respecting the size of different parts of the cord 

 is, that each part is in direct proportion to the size and number of 

 nerve-roots given off from it. Thus the cord is very large in the 

 middle and lower part of its cervical portion, whence arise the largo 

 nerve-roots for the formation of the brachial plexuses and the supply 

 of the upper extremities ; it again enlarges at the lowest part of its 

 dorsal portion and the upper part of its lumbar, at the origins of the 

 large nerves which, after forming the lumbar and sacral plexuses, are 

 distributed to the lower extremities. The chief cause of the greater 

 size at these parts of the spinal cord is increase in the quantity of 

 grey matter; the white part of the cord (especially the lateral 

 columns) becomes gradually and progressively smaller from above 

 downwards, because a certain number of fibres coming down from the 

 brain pass into the spinal grey matter at different levels. 



Grey matter. The grey matter of the cord consists of nerve- 

 fibres, most of which are very fine and delicate, of nerve-colls with 

 branching processes, and of an extremely delicate network of the 

 primitive fibrillse of axis -cylinders and of dendrites. This fine plexus 

 is called Qerlactis network, and is mingled with the meshes of 

 neuroglia. The neuroglia of the grey matter resembles that of the 

 white, but instead of everywhere forming a close network to support 

 the nerve-fibres, here and there it is in the form of a more open 

 sponge- work to support the nerve-cells. It is especially developed 

 around the central canal, which is lined with columnar ciliated 

 epithelium, the cells of which at their outer end terminate in fine 

 processes, which join the neuroglia network surrounding the canal, 

 and form the substantia gelatinosa centralis. It is also developed at 

 the tip of the posterior cornu of grey matter, forming what is known 

 .as the substantia gelatinosa lateralis of Kolando, which is much 

 enlarged in the upper cervical region. 



Groups of cells in the grey matter. The multipolar cells of the 

 grey matter are either scattered singly or arranged in definite 

 groups. 



(1) Anterior horn cells. In the cervical and lumbar enlargements 

 there are several groups of large multipolar cells in the anterior 

 horn ; in the thoracic region these are reduced to two, a mesial and a 

 lateral group. The larger groups correspond with segments of the 

 limbs, and in the cervical cord there is one special group from which 

 the phrenic nerve arises for the supply of the diaphragm. Tho axons 

 pass out by the anterior nerve-roots of the same side, but a few axons 

 pass to the antero-lateral column of the same side, and by the white 



-commissure to that of the opposite side. In birds, a few axons are 

 stated to pass to the posterior roots. 



(2) Posterior vesicular column of Lockhart Clarke ; generally known 

 as Clarke's column. This is a group of large nerve-cells with their long 



